1. Field of the Invention
The present invention relates to a semiconductor memory, and more particularly, to a semiconductor memory having a memory cell portion constituted by a plurality of memory arrays.
2. Description of the Related Art
Recently, semiconductor memories (for example, dynamic random access memories: DRAMs) have been strongly required to be highly integrated. Namely, a size of a transistor (memory cell) included in the DRAMs must be minimized and a gate oxide film of the transistor (MOS transistor) must also be thin.
By the way, recently, in semiconductor memory devices, a plurality of redundant memory cells are previously provided (manufactured), and defective memory cells (which may be caused in the manufacturing processes, and the like) are replaced by the redundant memory cells, so that a semiconductor memory having defective memory cells can be sold as a complete (fully operational) semiconductor memory.
However, when the number of the defective memory cells included in the semiconductor memory is larger than that of the redundant memory cells provided in the semiconductor memory, all of the defective memory cells cannot be replaced by the redundant memory cells, and the semiconductor memory cannot be sold as a complete operational semiconductor memory, but this semiconductor memory must be scrapped as a defective semiconductor memory.
In recent years, there has been studied and proposed a semiconductor memory having a memory cell portion (constituted by a plurality of memory arrays) which is divided into to two blocks, and when all half blocks of the memory arrays (half block of the memory cell portion) are normal (completely accessible), even though the number of the defective memory cells included in the semiconductor memory is larger than that of the redundant memory cells provided in the semiconductor memory, the semiconductor memory can be sold as a half good memory (or half capacity memory).
However, when all defective blocks of the memory cell portion of the semiconductor memory do not exist on one (one side) of the two memory blocks (which constitute each memory array of the memory cell portion), the semiconductor memory can not be sold as a half good memory (partial good memory), but is scrapped as a defective semiconductor memory.
The problems of the prior art will be explained hereinafter in detail with reference to the accompanying drawings.